This invention relates to an apparatus and method for driving an optical printer having at least one liquid crystal optical switch using a ferroelectric liquid crystal and which enables obtaining a high contrast.
In a conventional method of driving a ferroelectric liquid crystal optical switch array disclosed in U.S. Pat. No. 4,508,429, U.S. Pat. No. 4,591,886, U.S. Pat. No. 4,712,876 and U.S. Pat. No. 4,655,561, one frame term T.sub.L consists of a writing period T.sub.W for determining the light transmission state and a retention period T.sub.H (non-selection period in a time sharing driving operation) as shown in FIG. 2a. It takes about 0.1-0.5 ms for the liquid crystal molecules to respond to the applied voltage signal. Accordingly, the light transmission waveforms in ON state and OFF state in one frame term become as shown in FIG. 2b. The light transmission waveform OFF(A) represents a change of a light transmitting term into a light interrupting term, the light transmission waveform OFF(B) represents a change of a light interrupting term into a light interrupting one, the waveform ON(A) represents a change of a light interrupting term into a light transmitting term, and the light transmission waveform ON(B) represents a change of a light term into a light transmitting term. Thus, these light transmission waveforms have two kinds of waveforms, the light transmission states (ON) and light interrupting state (OFF) having different light transmission amounts (which are proportional to the areas of the hatched portions of FIG. 2b) respectively in one frame term. Out of these periods, the period ON(B) has a maximum light transmission amount. In the OFF periods, the light transmission amount of the period OFF(A) becomes higher. Consequently, the contrast ratio, i.e. the value of (light transmission amount in the ON periods) / (light transmission amount in the OFF periods) is not uniform. The contrast ratio is improved by reducing the light transmission amount in the period OFF(A). In this method, if the last period of one frame term is always put in the light interrupting state, the initial light transmission in the period OFF(A) can be eliminated.
The present inventors proposed a similar operation in U.S. Pat. No. 4,746,196 entitled "Multiplexed driving method for an optical switching element employing ferroelectric liquid crystal".
A time sharing driving method for a ferroelectric liquid crystal display panel, which has a writing reset period and a period in which the light transmission state of the reset is retained or reversed to select light transmitting state, is known (Japanese Patent Laid-Open No. 56937/1987). FIG. 3 shows the voltages at the scanning electrodes, the voltage at the signal electrodes and a voltage representing a difference between these two voltages, that is to say, which is an applied voltage to a liquid crystal layer, and a waveform of the relative transmitted light in this driving method. In a reset period 2 T.sub.L /N (wherein N is the number of time division; and T.sub.L is one frame term), a reset voltage signal V.sub.reset for obtaining a reset state (light interrupting state) is applied. This voltage signal consists of a combination of a pair of voltage pulses, i.e. +V.sub.0 /2 (portion A) in the first half T.sub.L /N of the reset period and -V.sub.0 (portion B) in the latter half T.sub.L /(2N) so that an average voltage in the reset period become substantially zero. Owing to this reset period, the contrast can be improved. Unless an average voltage in the reset period becomes substantially zero, the liquid crystal material is deteriorated, and this is why such a reset voltage is used.
In the above-described driving method, the voltage +V.sub.0 /2 (portion A in FIG. 3) is applied in the reset period, so that light is transmitted even in an OFF period. In case of a display, the number of time division is as large as several hundred, and the reset period substantially becomes shorter, so that such a degree of leakage of the light during the OFF period is negligible. However, in case of a liquid crystal optical switch for a printer, in which the number of time division is around ten at largest, the leakage amount of the light in one frame term becomes relatively high. This causes the contrast ratio to decrease, and the quality of the printed materials of the printer to become low. Therefore, this driving method cannot be adopted as the liquid crystal optical switch for a printer.